TT. H, Pennmg — On Concretions, 219 



unnoticed. This is a gradual passage from soft iron-stained sand to 

 hard complete concretions, exhibiting the various stages in the pro- 

 cess of their formation. This may be best observed toward the north 

 end of the cutting, in immediate proximity to and generally beneath 

 the patches of fully-formed concretions, but is visible also in other 

 parts of the section. 



The first step seems to have been the separation of the iron 

 particles into very thin lines resembling stratification, followed by 

 the union of two or more lines, by attraction or chemical aggrega- 

 tion acting on the iron. These thin layers of darker colour retain 

 their true linear arrangement until about one-eighth part of an inch 

 in thickness, when the space between them is reduced to about 3 or 

 4l inches, and they follow the lines of true or false -bedding as the 

 •case may be. They then begin to incline towards each other at 

 intervals varying from four inches to a foot or more in length, and 

 eventually the bent parts meet, thus enclosing irregular lenticular 

 patches of light-coloured sand. This is as seen in section parallel to 

 the line of cutting, but similar appearances would doubtless be pre- 

 sented in a transverse direction, and the lines of ironsand enclose 

 small masses of material, from which the iron has been removed. In 

 hardness and friability, and in all respects save colour, due to the 

 presence or partial absence of iron, the enclosing and enclosed 

 portions are the same, and the mass can be scraped with equal 

 readiness throughout. But the darker shell gradually hardens and 

 thickens by further elimination of iron from the neighbouring sand, 

 both within and around it, but mainly from within, for the harder 

 the shell has become, the lighter in colour is the nucleus, until it is 

 found almost a pure white in the complete concretion. 



These concretionary lumps may be seen in every stage, and in 

 gradations of every stage — in the earlier the sand presents no altera- 

 tion except a slight change in colour, then the shell hardens some- 

 what and stands out slightly from the face through weathering. It 

 becomes harder still, and some force is required to break it ; lastly it 

 passes into its hardest and thickest form when it can be broken only 

 by the hammer, and when it has removed all iron from the nucleus, 

 leaving it perfectly soft and white. 



The white central portion occurs in many forms, from a perfect 

 sphere to a flattened lenticular lump, sometimes it fills a pipe of the 

 hard ferruginous sand by which two larger masses have been con- 

 nected. It would not have been possible for these rounded masses of 

 white very loose sand to have been deposited as such, and for the iron- 

 stained sand to have aggregated round them — moreover, the hard 

 dark case is of a fairly uniform thickness, which would have been 

 equally impossible under the circumstances. The white enclosed 

 sand is loose from extraction of a portion of its original substance, 

 and under a lens it is seen to consist chiefly of white quartz. There 

 are some grains of the same mineral pink and yellow in colour, also 

 occasional specks of a black substance which most probably is 

 Glauconite. 



This method of origination fully accounts for the hollowness of, 



